It is known to provide joint prostheses having shafts adapted to be received in the bone cavity of a bone of a particular individual by ascertaining the geometry of that bone cavity and fabricating an individual prosthesis to fit the cavity. In general, these techniques have used a modification of a surface region of a shaft having a relatively large core filling most of the cross section of the joint cavity.
In German Patent Document DE-OS 37 40 438 and in the copending application mentioned above, a preformed shaft is described which is coated with a bone replacement or synthetic bone material, e.g. hydroxyapatite.
This coating can be applied in accordance with the geometry of the bone passage and the coating of the synthetic bone material can be of substantially greater thickness in the loading regions than is the case in regions of reduced loading.
Where the regions of greater loading lie for a particular patient in whom the joint prosthesis is to be implanted, can be determined without considerable effort utilizing X-ray imaging and with the aid of modern computer-supported mechanics. For example, computer tomography, which has been quite common of late in medicine, may be used to advantage for this purpose. The preformed shaft can be a serial or mass-produced product whose geometry follows average values for particular bones. For example, from a number of such shafts for differing bone sizes, a relatively good fitting shaft can be selected.
Of course, it is also possible to form the shaft individually, i.e. in accordance with measurements of the inner periphery and the cross section of the bone cavity into which the shaft is to be inserted.
Notwithstanding the efforts made heretofore, it has been found that singular stress regions arise in joint prostheses made by earlier methods and even utilizing the coated shaft of the above-identified applications and patents.